Testing of circuits is an essential step in the manufacture of high quality and reliable electronic products. The cost of an electronic product is related to the cost of the tests and the time necessary to generate and apply these. In terms of testing, mixed analogue and digital circuits, so called mixed-signal circuits, can be particularly time consuming and costly. Indeed, it has been reported that one of the greatest challenges in the coming years is the development of low-cost automatic test equipment for testing mixed-signal integrated circuits, see “International Technology Roadmap for Semiconductors”, 1999, published by the Semiconductor Industry Association (SIA).
Analogue signals are continuous as a function of both time and amplitude. Therefore the amount of information to be processed during testing is potentially very large. Unlike digital systems, interpreting whether an analogue output signal does or does not actually indicate a fault can be ambiguous. This inability to discriminate is made worse by the greater functionality and complexity of even the simplest analogue circuit, particularly if input signals are limited in complexity or time duration. In addition, because output signals are analogue, there can be no ideal performance from a completely “correct” circuit. Instead all assessment must be based on the concept that each component is subject to tolerances, which contribute towards variability in system behaviour even under fault-free conditions. All of these features make the testing of analogue circuits somewhat problematic.
One of the main problems with testing mixed-signal circuits is the need for separate analogue and digital test instruments. Over the past few years, a number of approaches have been proposed to unify the test method for mixed-signal systems. These approaches include power supply monitoring, digital modelling of analogue circuits and the use of digital test signals. As regards the use of digital signals, a number of techniques have been suggested, for example step response testing. This is described by Souders et al in the article “Accurate Frequency Response Determinations from Discrete Step Response Data”, IEEE-Trans. on Instrumentation and Measurement, Vol. IM-36, No. 2, pp. 433–9, June 1987. Testing using a complementary signal set is another proposed method for digital testing of analogue circuits. This is described by Eckersall et al in the article “Testing an Analogue Circuit using a Complementary Signal Set”, IEE Colloquium on “Testing Mixed Signal Circuits”, Digest No. 1992/118, pp. 5/1–5/6, 1992. Pseudorandom testing is yet another option. Examples of this are described by: a) Al-Qutayri et al in “Go/No-Go Testing of Analogue Macros”, IEE Proc. Circuits, Devices and Systems, Vol. 139, No. 4, pp. 534–540, August 1992; b) Pan et al in “Pseudorandom Testing for Mixed-Signal Circuits”, IEEE Trans. on Computer-Aided Design of Integrated Circuits and Systems, Vol. 18, No. 10, pp. 1173–1185, October 1997, and c) Variyam et al in “Digital-Compatible BIST for Analog Circuits Using Transient Response Sampling”, IEEE Design & Test of Computers, pp. 106–115, July–September 2000.
Existing digital approaches for testing mixed-signal systems have had limited success. Nevertheless, because of the advantages of using a digital signal to test mixed-signal circuits, significant and continuing efforts are being made to investigate this. However, despite extensive research, a satisfactory solution to the problem of how to test analogue and/or mixed-signal circuits using a digital signal has not been found.
An object of the invention is to provide an improved method and system for digital testing of analogue and mixed-signal circuits.